The sulfur content in iron ores and other materials commonly used in pig-iron production, i.e. coal and coke, have increased the costs of steel making. As a result, it is becoming increasingly desirable to desulfurize the pig-iron before the iron enters the basic oxygen furnace and/or steel making furnace. In addition, specifications for the sulfur content of finished steel are decreasing to extremely low levels to make high strength low alloy steel, and steels resistant to hydrogen induced cracking, among other applications requiring low sulfur contents. In combination with the economic benefits of blast furnace operations producing molten pig-iron with increased sulfur contents, the desulfurization of molten pig-iron external to the blast furnace before the molten pig-iron enters the steel making furnace has become a practical necessity.
Over the years, a wide variety of materials and mixtures have been used to desulfurize pig-iron. It has long been known that various calcium compounds are good desulfurization agents. It has also been known that magnesium, alone or in combination with various alkaline metal oxides, is also a good desulfurization agent. There have been several patents which disclose the use of calcium oxide and magnesium as the primary desulfurization agents. (See Skach U.S. Pat. No. 4,765,830; Skach U.S. Pat. No. 4,708,737; Green U.S. Pat. No. 4,705,561; Kandler U.S. Pat. No. 4,139,369; Kawakami U.S. Pat. No. 4,137,072; Koros U.S. Pat. No. 3,998,625.) Furthermore, desulfurization agents disclosing the use of calcium carbide as the primary desulfurization agent have also been known and well documented. (See Freissmuth U.S. Pat. No. 3,598,573; Todd U.S. Pat. No. 3,929,464; Braun U.S. Pat. No. 4,395,282.)
The use of a desulfurization agent that includes magnesium and iron carbide or high carbon ferromanganese is disclosed in Luxemburg Patent No. 88,252 dated Jan. 3, 1999 and invented by Axel Thomas. The desulfurization agent disclosed in Thomas '252 includes a majority of iron carbide or high carbon ferromanganese. The desulfurization agent also includes magnesium, and one or more additives to improve the formed slag. The particles of iron carbide or high carbon ferromanganese are selected to be the same or slightly greater in size than the particles of magnesium. The particle sizes of the iron carbide or high carbon ferromanganese and magnesium range from 0.5 to 1 mm. As a result, the particles of iron carbide or high carbon ferromanganese do not coat the particles of magnesium, or vice versa. The iron carbide or high carbon ferromanganese and magnesium can be coated with titanium oxide to improve the fluidity of the particles and to slow the melting rate of the particles. The iron carbide or high carbon ferromanganese and magnesium can be mixed together prior to injection into the pig-iron or injected separately into the pig-iron.
The use of a calcium compound and/or magnesium, in combination with a gas-producing compound, has also been used to increase the amount of sulfur removal. It has been found that the gas-producing compound releases a gas upon contact with the molten pig-iron to create a turbulent environment within the molten pig-iron. The released gas primarily breaks down agglomerations of the desulfurization agent and disperses the desulfurization agent throughout the molten pig-iron. The gas-producing agent is typically a hydrocarbon, carbonate or alcohol which has a tendency to release various amounts of gas upon contact with the molten pig-iron. Use of these various gas-producing agents is well documented. (See Takmura U.S. Pat. No. 3,876,421; Meichsner U.S. Pat. No. 4,078,915; Gmohling U.S. Pat. No. 4,194,902; Koros U.S. Pat. No. 4,266,969; Freissmuth U.S. Pat. No. 4,315,773; Koros U.S. Pat. No. 4,345,940; Green U.S. Pat. No. 4,705,561; Rellermeyer U.S. Pat. No. 4,592,777; Meichsner U.S. Pat. No. 4,764,211; Meichsner U.S. Pat. No. 4,832,739; and Luyckx U.S. Pat. No. 5,021,086.)
The use of compounds to increase the desulfurization efficiencies of magnesium particles are disclosed in Bieniosek U.S. Pat. No. 6,352,570 and Bieniosek U.S. Pat. No. 6,372,014. High melting temperature particles are combined with and/or coated onto the magnesium particles to delay the melting of the magnesium particles.
Desulfurization agents can contain various slag-forming agents. The importance of the slagging agent generally has been passed over for more immediate concerns about the economics of using various ingredients of the desulfurization agent. The composition of the slag can be important to retain the removed sulfur within the slag and not allow the sulfur to re-enter the molten pig-iron. Various slagging agents have been used for various purposes. In U.S. Pat. Nos. 4,315,773 5,021,086, fluorspars are used to modify the characteristics of the slag produced during the desulfurization process.
Many of the above described desulfurization agents remove the desired amount of sulfur and other impurities from molten iron. However, in an industry constantly driven by margins, there remains a need for a more cost effective desulfurization agent. The magnesium component of the desulfurization agent is typically the highest-cost component. Domestically refined primary magnesium powder can cost up to $1.80/lb. As a result, there has been some interest in using magnesium scrap. Magnesium scrap is available from rejected and process scrap in the form of machined chips which are common in the automobile and electronics industry. Magnesium metal is commonly machined using mineral oil and oil/water emulsions resulting in waste magnesium chips and cutting fluid. The cutting fluid can constitute up to 35–50 weight percent of the waste material. The magnesium chip/cutting fluid mixture typically cannot be disposed of due to the reactivity of magnesium with water. The large volume of cutting fluid in the magnesium chip/cutting fluid mixture increases the transportation costs of the mixture. Due to the transport costs and/or processing problems of the magnesium chip/cutting fluid mixture, the mixture is commonly burned instead of being reclaimed.
Some progress has been made concerning the recovery of magnesium from a magnesium chip/cutting fluid mixture. Several of these processes are disclosed in U.S. Pat. Nos. 2,299,043; 2,358,667; 3,656,735; 3,767,179; and 5,338,335. In these processes, the water and oil in the magnesium chip/cutting fluid mixture is burnt off in a rotary kiln. The substantially oil free magnesium chips are then remelted and formed and/or extruded into a final product. Solvents may be used to separate a portion of the cutting fluid from the magnesium chips prior to drying the magnesium chips. Although these processes are successful in reclaiming magnesium, the energy costs associated with the heating of the magnesium chip/cutting fluid mixture have not resulted in a cost effective process. Combustion problems remain with the drying of the magnesium chips resulting in higher recovery costs. In addition, the oxidation of the magnesium during the drying process accounts for a significant loss of magnesium being reclaimed. Additional losses are encountered when using a solvent prior to drying.
Another process for reclaiming magnesium from a magnesium chip/cutting fluid mixture is by pressing the mixture together to form a magnesium puck or briquette. This process can reduce the cutting fluid content of puck or briquette to about 7%. The squeezed out cutting fluid can be recycled and the transport costs of the magnesium in the form of a puck or briquette is significantly reduced. In addition, due to the low cutting fluid content of the puck or briquette, the puck or briquette can be more safely transported in such form. Furthermore, the compression process is less costly than processing utilizing a heated rotary kiln. Although the compression process has several cost advantages, the cutting of fluid content of up to 7% poses problems for further processing of the compressed magnesium chips. Smelting of the magnesium pucks or briquettes is not feasible because of extreme flame and emissions generation. As a result, magnesium pucks or briquettes have not been accepted in the industry. In addition, the magnesium pucks or briquettes could not be disposed of in land fills due to environmental and safety concerns.
In view of the present state of technology, there is a need for a lower cost and effective desulfurization agent that can utilize reclaimed magnesium from a magnesium chip/cutting fluid mixture.